1. Field of the Invention
The present invention relates to a process for the preparation of a molding material modified with acrylic rubber, by graft polymerization of monomers forming the graft shell, in the presence of an acrylic rubber which is swollen or dissolved in the monomers and contains macromonomers as comonomers.
2. Description of the Related Art
The preparation of rubber-modified molding materials has long been known. The use of elastomeric acrylate polymers having glass transition temperatures of less than 0.degree. C. and preferably less than -10.degree. C. (acrylic rubber), for molding materials modified in this manner has also been known for almost 40 years. Compared with the molding materials prepared with diene rubbers, they have improved weathering resistance. In the modified multiphase molding materials, domains of the rubber are embedded in a matrix of a thermoplastic, the domain structure playing a major role in determining the mechanical properties of the resulting molding materials. The toughness of the molding materials results from increased energy absorption during deformation up to fracture, energy being consumed for the formation of microcavities or for inducing sliding processes of the matrix polymer chains. The multiphase character is therefore an essential precondition for achieving high impact strengths.
There is still a great need for molding materials modified with acrylic rubber and in particular for styrene/acrylonitrile copolymers modified with acrylic rubber (ASA molding materials), which have good weathering resistance, high impact strength and good flow properties.
The preparation of ASA molding materials in emulsion is described in many publications in the patent literature (cf. for example DE-A 19 11 882, DE-A 28 26 925, DE-A 31 29 378, DE-A 31 29 472, DE-A 31 49 046, DE-A 31 49 358, DE-A 32 06 136, DE-A 32 27 555).
The disadvantage of this preparation in emulsion is the requirement for the removal of assistants during the working up of the molding materials, in order to avoid subsequent problems during their processing (discoloration, speck formation, corrosion). In particular, however, the impact strength, tensile strength and gloss properties of the surfaces of shaped articles produced therefrom are unsatisfactory.
DE-B 11 82 811, published more than 30 years ago, discloses the polymerization of an acrylate together with a crosslinking monomer in solution for the preparation of a rubber-modified molding material, the monomers styrene and acrylonitrile to be grafted on being added after a conversion of only from 20 to 40% by weight of the monomers and then being polymerized by mass or solution polymerization. The varying composition in the graft reaction and incorporation of rubber units into the graft shell by polymerization result in a reduction of the Vicat softening temperature and a deterioration in other mechanical properties of the resulting molding material.
It is an object of the present invention to prepare molding materials which are modified with acrylic rubber and can be processed to give shaped articles having improved impact strength and good flow properties and reduced surface gloss.